The human spine comprises individual vertebras 30 (segments) that are connected to each other to form a spinal column 29, shown in FIG. 1. The vertebras 30 are separated and cushioned by thin pads of tough, resilient fiber known as inter-vertebral discs 40. Disorders of the spine occur when one or more of the individual vertebras 30 and/or the inter-vertebral discs 40 become abnormal either as a result of disease or injury. In these pathologic circumstances, fusion of adjacent vertebral segments may be tried to restore the function of the spine to normal, achieve stability, protect the neural structures, or to relief the patient of discomfort.
Several spinal fixation systems exist for stabilizing the spine so that bony fusion is achieved. The majority of these fixation systems use rods 130, 140 that attach to screws 138, 136 inserted into the vertebral bodies, shown in FIG. 2. For spinal fixation in the cervical spine area 28, the proximal ends 151, 152 of the rods 130, 140, are molded to fit the anatomy of the skull 50 and the cervical spine 28 and are attached to an occipital fixation plate 150 that is implanted in the occiput 26.
Occipital fixation plates currently available are T-shaped, Y-shaped, or horseshoe-shaped, as shown in FIG. 2. All these types of occipital fixation plates have fixed geometrical shape and dimensions. In particular, the distance 135 between the lower ends of the plate is fixed, which requires that the rods 140, 130 are contoured during surgery so that they fit the anatomy of the patient. In general, most of the rod fixation systems require contouring of each rod across several vertebras in many cases. The contouring of each rod depends on the configuration of the screws and varies from side to side in the same patient and among patients. This contouring process may add considerable time to the surgery. Recent generations of screws and rod connectors seek to diminish this drawback by allowing variable axes of movements in the screw recess for the rod or in the rod connectors. However, in most cases this adds another level of complexity to the operation and often further increases the operative time. This increase in operative time and the complexity of the connectors put substantial stress on the surgeon and the supporting staff. Even in the hands of the best spine surgeon, the rod is often not perfectly contoured to align with the screws. Hence the surgeon has to use substantial force at multiple points along a rod to hold the rod to the screws or connectors while counteracting the adjacent soft tissues. This maneuver risks soft tissue damage and also puts the dura and the neural contents at risk for dural tears or spinal cord or nerve damage if a holding instrument slips.
A spine fixation assembly that utilizes plates instead of rods is described in U.S. Pat. No. 6,626,909, the contents of which are incorporated herein by reference. Referring to FIG. 3, a plate spine fixation assembly 600 connects adjacent vertebrae 92, 94 and 96. The spine fixation assembly 600 includes plates 610 and 612 which are placed diagonally to each other and transverse plates 614, 616. Plates 610 and 612 are attached to diagonally opposite pedicle screws and are cross-coupled at midpoint 608 forming an X-structure. The top and bottom pedicles 92A, 92B and 96A, 96B are connected with transverse plates 614 and 616, respectively. The basic X-shape structure may be repeated to extend the spine fixation in either caudad 672 or cephalad 670 directions. The modular structure of the spine fixation assembly 600 allows a surgeon to correct spinal deformities over any distance and orientation along the entire spine 29. However, for certain spinal locations, such as the occiput, rod or a combination of rods and plates may be preferred. Furthermore, there is a need for an occipital fixation assembly that has adjustable width and does not require contouring of elements during surgical implantation.